The invention relates to turning tandem bogie wheels of a rubber-tyred crane by means of one drive mechanism. More precisely, the invention relates to a crane comprising bogie structures in the corners of the crane, two wheels arranged in tandem in each bogie structure, the wheels being arranged in the bogie structure to be turnable about the vertical axis, the wheels being offset from the vertical axis, one wheel being a drive wheel and the other an idle wheel, a mechanism that drives and turns each drive wheel, and a turning device arrangement provided between each wheel pair for turning the wheel pair in co-operation to a selected position.
A gantry crane type that is generally used in container freight terminals is a powered rubber-tyred gantry (RTG) crane. Cranes come in a variety of types and sizes; generally, the classification is based on the number of wheels in the crane, i.e. 4-, 8- or 16-wheeled cranes. The crane has various operating modes, i.e. directions of movement: normal, lateral and circling. In the normal, common operating mode, the crane wheels are perpendicular to the driving direction of the hoist carriage of the crane. This mode enables the crane to move in the direction of a container stack. In the lateral mode the crane wheels are turned to be in parallel with the driving direction of the hoist carriage, and consequently it is possible to move the crane between the container stacks. In the circling mode the crane wheels are turned tangentially to the crane's centre (radius of turn). In this mode it is possible to rotate the crane horizontally about its central axis.
There are various wheel turning arrangements and two basic principles apply in the most commonly used ones. In the arrangement in accordance with the first basic principle, crane wheels are turned with a separate turning mechanism, typically a hydraulic cylinder or a separate turning apparatus. The wheels are placed in a centralized manner with respect to the radius of turn and the wheel is turned on the spot. It is known that the arrangement concerned requires great force and wears the wheel considerably. According to the second basic principle, the driving mechanism of the crane is used for turning the wheels, and in that case the wheel is offset from the turning centre and turns rolling.
To turn crane wheels by means of the drive mechanism is known, for instance, from publications FI 101957 B and JP 2001-213587 A and JP 2002-137887 A.
Publication FI 101957 B discloses a bogie wheel arrangement, in which one bogie comprises two wheels side by side, one of the wheels being a drive wheel and the other an idle wheel. Each bogie comprises a specific drive mechanism and consequently a specific turning mechanism.
Publications JP 2001-213587 A and JP 2002-137887 A disclose a bogie arrangement, in which there are two bogies, both having two wheels side by side. The drive mechanism is only arranged in one bogie, and for turning the bogies are interconnected with a turning rod or the like and a mechanism providing a turning motion, generally a hydraulic cylinder.
Now, it is desired that the corresponding turning principle be applied to an 8-wheel crane, for instance, in which one bogie comprises two tandem wheels. The arrangement concerned poses a problem, however, that in order to provide all three operating modes the position of the wheels in different modes is not viable or optimal. If the connecting bar between the wheels is cross-coupled on different sides of the turning centre of the wheels, the wheels can be turned in a transverse mode symmetrically at the bogie ends, whereby loading will be evenly distributed on both wheels. However, this does not allow a circling mode arrangement, because the wheels cannot be turned in the same direction. If the connecting rod is coupled on the same side of the turning centres, a circling mode will work, but in a transverse position the wheels are asymmetrical with respect to the bogie centre. For instance, the solution of the above-mentioned JP-publications does not provide any help in turning two tandem wheels placed asymmetrically with respect to the turning centre in the bogie, because in the arrangement of the JP publications the wheels of each bogie are placed symmetrically with respect to the turning centre, and thus the above-mentioned symmetry-related problem does not occur.